A synthetic approach to pseudoproline dipeptides is published in PCT Publication WO 2008/000641, wherein access to the compound of formula I is accomplished via an ammonium salt intermediate of formula
wherein further R1, R2, R5, R6, R7 and R8 are as defined in PCT 2008/000641.
One major disadvantage of the PCT 2008/000641 approach is the need to purify the dipeptide by isolation of its ammonium salt intermediate, which prior to the ring closure has to be liberated to the dipeptide. Accordingly this synthesis is less suitable for application on a technical or commercial scale.
The present invention overcomes the disadvantages of the PCT 2008/000641 approach and instead provides a short and technically feasible synthesis of the pseudo proline dipeptides of formula I which allows for obtaining the product with a high yield.